Acoustic wave sensors are utilized in a variety of sensing applications, such as, for example, temperature and/or pressure sensing devices and systems. Acoustic wave devices have been in commercial use for over sixty years. Although the telecommunications industry is the largest user of acoustic wave devices, they are also used for chemical vapor detection. Acoustic wave sensors are so named because they use a mechanical, or acoustic, wave as the sensing mechanism. As the acoustic wave propagates through or on the surface of the material, any changes to the characteristics of the propagation path affect the velocity, phase and/or amplitude of the wave.
Changes in acoustic wave characteristics can be monitored by measuring the frequency, phase characteristics, Q value, insertion loss and input impedance of the sensor and can then be correlated to the corresponding physical quantity or chemical quantity that is being measured. Virtually all acoustic wave devices and sensors utilize a piezoelectric crystal to generate the acoustic wave. Three mechanisms can contribute to acoustic wave sensor response, i.e., mass-loading, visco-elastic and acousto-electric effect. The mass-loading of chemicals alters the frequency, amplitude, and phase and Q value of such sensors. Most acoustic wave chemical detection sensors, for example, rely on the mass sensitivity of the sensor in conjunction with a chemically selective coating that absorbs the vapors of interest resulting in an increased mass loading of the acoustic wave sensor. The chemically sensitive coating, however, significantly reduce the Q value, hence the performance of the sensor. In a practical sensor design, the acoustic wave sensor is preferred to be as simple as possible (in electrode configurations, layers of electrode, coatings on top of the electrodes, etc).
Examples of acoustic wave sensors include acoustic wave detection devices, which are utilized to detect the presence of substances, such as chemicals, or environmental conditions such as temperature and pressure. An acoustical or acoustic wave (e.g., SAW/BAW) device acting as a sensor can provide a highly sensitive detection mechanism due to the high sensitivity to surface loading and the low noise, which results from their intrinsic high Q factor. Bulk acoustic wave devices are typically fabricated using a vacuum plater, such as those made by CHA, Transat or Saunder. The choice of the electrode materials and the thickness of the electrode are controlled by filament temperature and total heating time. The size and shape of electrodes are defined by proper use of masks. Surface acoustic wave devices are typically fabricated using photolithographic techniques with comb-like interdigital transducers placed on a piezoelectric material. Surface acoustic wave devices may have either a delay line or a resonator configuration.
Based on the foregoing, it can be appreciated that acoustic wave devices, such as a surface acoustic wave resonator (SAW-R), surface acoustic wave delay line (SAW-DL), surface transverse wave (STW), bulk acoustic wave (BAW), can be utilized in various sensing measurement applications. One of the primary differences between an acoustic wave sensor and a conventional sensor is that an acoustic wave can store energy mechanically. Once such a sensor is supplied with a certain amount of energy (e.g., through RF), the sensor can operate for a time without any active part (e.g., without a power supply or oscillator). This feature makes it possible to implement an acoustic wave device in an RF powered passive and wireless sensing application.
One promising application for micro-sensors involves engine oil quality sensors. Acoustic wave viscosity sensors have been tested in the context of oil quality sensors. While viscosity is a good indicator of many oil quality factors, including low temperature startability, fuel economy, thinning or thickening effects at high/low temperatures, lubricity, and oil film thickness in running automotive engines, viscosity is not an indicator of how much acid is present in engine oil. Such acids can cause a lot of damage to an automotive engine.
A certain percentage of engine oil is converted to acids and other unhealthy compounds. Theses acids can attack the engines by etching the engines. The truck automotive industry, for example, currently relies on engine oil's pH, TBN and TAN as indicators of oil quality, however, such methods do not provide direct indications of precisely how much acid is attacking the engine. Combining a viscosity and corrosivity monitor might provide a technique for solving this problem. Such a method, however, is very costly, time consuming and takes up a great deal of volume.
Based on the foregoing, it is believed that what is needed to overcome these problems involves the implementation of an etch rate (corrosivity) monitor in combination with viscosity measurements. It is believed that the oil quality monitoring methods and systems described herein can solve such conventional deficiencies.